window.lua -- is responsible for building the luapdf reader window and
defining several helper methods (I.e. w:new_tab(path),
w:close_tab(), w:close_win(), etc).

document.lua -- is a wrapper around the document widget object and is
responsible for watching document signals (I.e.
"key-press", etc). This file
also provides several window methods which operate on the
current document tab (I.e. w:reload(),
w:back(), w:forward()).

modes.lua -- manages the modal aspect of the reader and the actions
that occur when switching modes.